Families of youth with OCD are characterized by an array of stressful family dynamics that predict poor response to cognitive behavior therapy (CBT), the current treatment of choice for pediatric OCD. However, understanding of the specific mechanisms which family functioning interferes with treatment is limited. Youngsters in unstable home environments have been found to exhibit prolonged activation of stress response systems, and separate lines of research link this heightened reactivity to disrupted extinction learning and the maintenance of fear and avoidance behaviors. Thus, one possibility is that poor family functioning complicates specific tasks of CBT (e.g., exposure exercises, family disengagement from symptoms) via heightened stress reactivity. Building on an existing K23-funded clinical trial, this study examines parent and child stress reactivity and its role in the treatment process for pediatric OCD. Employing an integrated translational research battery, the proposed project examines parent and child stress reactivity over the course of CBT with the goal of understanding (a) how the pathophysiology of pediatric OCD changes with treatment; (b) how parent and child stress reactivity relate to ability to complete key tasks of treatment; and (c their role in treatment outcome. Stress reactivity is assessed via multiple methods, including measures of hypothalamus- pituitary-adrenal (HPA) axis reactivity (salivary cortisol) as well as established markers of sympathetic nervous system activity (salivary amylase, heart rate and skin conductance) during baseline and exposure tasks. Findings from this work have the potential to shed light on the biological underpinnings of pediatric OCD, to improve understanding of the mechanisms treatment non-response, and to guide efforts to improve interventions for refractory subgroups.